Method and apparatus for transmitting/receiving multiple codewords in SC-FDMA system

ABSTRACT

The present invention relates a method and apparatus for transmitting/receiving data using multiple codewords in a communication system using SC-FDMA (single carrier frequency division multiple access). A transmitter generates the multiple codewords for user data and transmits the generated multiple codewords. A receiver receives the multiple codewords and sequentially performs decoding and SIC (successive interference cancellation) on the received multiple codewords. Therefore, this structure can minimize a PAPR (peak to average power ratio) and considerably reduces interference between symbols in a frequency selective fading environment.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/476,125, filed on Jun. 1, 2009, which is a continuation of PCTapplication No. PCT/KR2007/006142 filed Nov. 30, 2007, which claimspriority to and the benefit of Korean Patent Application No.10-2006-0120762 filed in the Korean Intellectual Property Office on Dec.1, 2006, and No. 10-2007-0057886 filed in the Korean IntellectualProperty Office on Jun. 13, 2007, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a method and apparatus fortransmitting/receiving multiple codewords in an SC-FDMA (single carrierfrequency division multiple access) system. More particularly, thepresent invention relates to a method and apparatus fortransmitting/receiving data using multiple codewords in a communicationsystem using SC-FDMA.

This work was supported by the IT R&D program of MIC/IITA[2006-S-001-01, Development of Adaptive Radio Access and TransmissionTechnologies for 4^(th) Generation Mobile Communications].

(b) Description of the Related Art

An OFDMA (orthogonal frequency division multiple access) system dividessubcarriers for multiple users, and is suitable to transmit/receive dataat a high speed through wire and wireless radio channels. Since theOFDMA system uses a plurality of carriers having orthogonalitytherebetween, efficiency in the usage of frequency is improved, andprocesses of modulating/detecting a plurality of carriers in atransmitter and a receiver obtain the same results as IDFT (inversediscrete Fourier transform) and DFT (discrete Fourier transform) areperformed on the carriers. Therefore, the OFDMA system can perform themodulating/detecting processes at a high speed using IFFT (inverse fastFourier transform) and FFT (fast Fourier transform).

Since the OFDMA system is suitable for high-speed data transmission andreception, it has been adopted as a standard scheme of an IEEE 802.11astandard, a high-speed wireless LAN (local area network) of a HIPERLAN/2system, a broadband wireless access (BWA) system of an IEEE 802.16standard, a digital audio broadcasting (DAB) system, a digitalterrestrial television broadcasting (DTTB) system, an ADSL (asymmetricdigital subscriber line), or a VDSL (very high data-rate digitalsubscriber line).

However, the OFDMA system has a problem in that a PAPR (peak to averagepower ratio) is high. In order to solve this problem, an SC-FDMA (singlecarrier frequency division multiple access) system that extends anSC-FDE (single carrier with frequency domain equalization) system to asubcarrier division system has been proposed.

In order to obtain a single carrier characteristic while maintainingfrequency orthogonality, the SC-FDMA system is constructed by adding aDFT-spreading structure to the existing OFDMA system.

The SC-FDMA system enables multiple access using frequency domainorthogonality, which is an advantage of the existing OFDMA system, andcan reduce a PAPR, which has not been achieved by the existing OFDMAsystem. However, the SC-FDMA has a problem in that interference betweentransmission symbols occurs in a frequency selective fading environment,resulting in low performance.

As described above, since the SC-FDMA system transmits or receives onlyone stream, there is a limitation in removing the interference betweenthe symbols. As a result, the performance of the SC-FDMA system islowered due to the interference between the symbols in the frequencyselective fading environment.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the invention andtherefore it may contain information that does not form the prior artthat is already known in this country to a person of ordinary skill inthe art.

SUMMARY OF THE INVENTION

The present invention has been made in an effort to provide a method andapparatus for transmitting/receiving data using multiple codewords in acommunication system using SC-FDMA.

Further, the invention has been made in an effort to minimize a PAPR ina communication system using SC-FDMA and to considerably reduceinterference between symbols in a frequency selective fadingenvironment.

An embodiment of the present invention proposes a method oftransmitting/receiving data using multiple codewords in a communicationsystem using SC-FDMA and the structure of a transceiver fortransmitting/receiving data using multiple codewords in a communicationsystem using SC-FDMA.

According to an exemplary embodiment of the present invention, there isprovided a method of transmitting multiple codewords in an SC-FDMA(single carrier frequency division multiple access) system. The methodincludes dividing a user data stream into a plurality of sub-streams,independently performing channel coding and QAM (quadrature amplitudemodulation) mapping on each of the sub-streams, mapping a signalsubjected to the QAM mapping to a DFT (discrete Fourier transform)index, and performing DFT on the signal mapped to the DFT index andtransmitting the multiple codewords.

In the transmitting method, the transmitting of the multiple codewordsmay include performing DFT on the signal mapped to the DFT index,mapping a signal obtained by the DFT to a subcarrier corresponding tothe user, and performing IFFT (inverse fast Fourier transform) on thesignal mapped to the subcarrier and transmitting the multiple codewords.

The transmitting method may further include receiving channel stateinformation for each of the sub-streams from a receiver, and determininga code rate and a modulation method of each of the sub-streams andperforming the channel coding and the QAM mapping on the basis of theresult of the determination. The transmitting method may further includecalculating an SINR (signal to interference and noise ratio) of each ofthe sub-streams using information that is fed back from a receiver, anddetermining a data transfer rate on the basis of the calculated SINR andtransmitting the multiple codewords. The transmitting method may furtherinclude performing the channel coding and the QAM mapping on the basisof a data transfer rate that is fed back from a receiver, anddetermining the transmission power of each of the sub-streams on thebasis of the transmission power of a first sub-stream and thetransmission power offset between the sub-streams that are fed back fromthe receiver.

According to another exemplary embodiment of the present invention,there is provided a method of receiving multiple codewords in an SC-FDMA(single carrier frequency division multiple access) system. The methodincludes receiving the multiple codewords and performing FFT fastFourier transform) on the received multiple codewords, performing asubcarrier demapping process on signals obtained by the FFT to extractsignals corresponding to each user, and detecting and decoding the usersignals to estimate signals corresponding to the users.

In the receiving method, the estimating of the signal may include:detecting one sub-stream of each received user signal; performingdecoding using a plurality of detected output signals forming thesub-stream; determining a transmission signal for the sub-stream on thebasis of the decoded signal; multiplying the transmission signal by achannel value to generate a received signal for the sub-stream;subtracting the received signal for the sub-stream from the receiveduser signal to generate a modified user reception signal; and repeatedlyperforming the procedure from the detecting of the sub-stream to thegenerating of the modified user reception signal on the othersub-streams. In the receiving method, the estimating of the signal mayinclude checking whether a CR (cyclic redundancy) value of the decodedsignal is correct, and when the CR value is correct, performing thedetermining of the transmission signal.

The receiving method may further include calculating a post-detectionSINR (signal to interference and noise ratio) of each sub-stream inconsideration of SIC (successive interference cancellation) after thechannels of all of the sub-streams are estimated, and feeding back thecalculated SINR to a transmitter.

According to still another exemplary embodiment of the presentinvention, there is provided an apparatus for transmitting multiplecodewords in an SC-FDMA (single carrier frequency division multipleaccess) system. The apparatus includes: a demultiplexer thatdemultiplexes a user data stream into a plurality of sub-streams; aplurality of encoders that independently perform channel coding on thesub-streams; a plurality of QAM (quadrature amplitude modulation)mappers that independently perform QAM mapping on the signals subjectedto the channel coding; a DFT mapper that maps the signals subjected tothe QAM mapping to DFT (discrete Fourier transform) indexes; a DFT(discrete Fourier transformer) that performs DFT on the signals mappedto the DFT indexes; a subcarrier mapper that maps a signal obtained bythe DFT to a subcarrier corresponding to a user; and an IFFT (inversefast Fourier transformer) that performs IFFT (inverse fast Fouriertransform) on the signal mapped to the subcarrier and transmits themultiple codewords.

The transmitting apparatus may further include a rate controller thatreceives channel state information for each of the sub-streams from areceiver, determines a code rate and a modulation method of each of thesub-streams, and applies the determined code rate and modulation methodto each encoder and each QAM mapper.

According to yet another exemplary embodiment of the present invention,there is provided an apparatus for receiving multiple codewords in anSC-FDMA (single carrier frequency division multiple access) system. Theapparatus includes an FFT (fast Fourier transformer) that receives themultiple codewords and performs FFT (fast Fourier transform) thereon, asubcarrier demapper that performs a subcarrier demapping process onsignals output from the FFT to extract signals corresponding to eachuser, and a plurality of detectors and decoders that detect and decodethe user signals to estimate the signals corresponding to each user.

The receiving apparatus may further include a signal regenerator thatgenerates a transmission signal for each sub-stream using the decodedsignal, and an SIC (successive interference cancellation) device thatmultiplies the transmission signal by a channel value to generate areceived signal for the sub-stream, and subtracts the received signalfor the sub-stream from the received user signal to generate a modifieduser reception signal. The receiving apparatus may further include a CRC(cyclic redundancy check) device that checks whether a CR (cyclicredundancy) value of the decoded signal is correct, and when the CRvalue is correct, notifies the signal regenerator of the fact. Thereceiving apparatus may further include a channel state informationgenerator that calculates a channel state information value for thesub-stream and feeds back the calculated value to a transmitter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a transmitter for transmittingmultiple codewords in an SC-FDMA (single carrier frequency divisionmultiple access) system according to an exemplary embodiment of thepresent invention.

FIG. 2 is a flowchart illustrating a method of transmitting multiplecodewords in an SC-FDMA system according to an exemplary embodiment ofthe present invention.

FIG. 3 is a diagram illustrating an example of a mapping methodperformed by a DFT (discrete Fourier transform) mapper according to anexemplary embodiment of the present invention when M=L.

FIG. 4 is a diagram illustrating another example of the mapping methodperformed by the DFT mapper according to the exemplary embodiment of thepresent invention when M=L.

FIG. 5 is a diagram illustrating an example of a mapping methodperformed by a DFT mapper according to an exemplary embodiment of thepresent invention when M=L/2.

FIG. 6 a diagram illustrating another example of the mapping methodperformed by the DFT mapper according to the exemplary embodiment of thepresent invention when M=L/2.

FIG. 7 is a block diagram schematically illustrating the structure of areceiver for receiving multiple codewords in an SC-FDMA system accordingto an exemplary embodiment of the present invention.

FIG. 8 is a block diagram illustrating the structure of a receiver forreceiving multiple codewords for a user k in an SC-FDMA system accordingto another exemplary embodiment of the present invention.

FIG. 9 is a flowchart illustrating a method of receiving multiplecodewords in an SC-FDMA system according to an exemplary embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, exemplary embodiments of the present invention will bedescribed with reference to the accompanying drawings such that thoseskilled in the art can easily realize the invention. As those skilled inthe art would realize, the described embodiments may be modified invarious different ways, all without departing from the spirit or scopeof the present invention. Accordingly, the drawings and description areto be regarded as illustrative in nature and not restrictive. Likereference numerals designate like elements throughout the specification.

It will be understood that the terms “comprises” and/or “comprising,”when used in this specification, specify the presence of statedfeatures, integers, steps, operations, elements, and/or components, butdo not preclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof.

Hereinafter, a method and apparatus for transmitting/receiving multiplecodewords in an SC-FDMA system (i.e., a communication system usingSC-FDMA) according to an exemplary embodiment of the present inventionwill be described in detail with reference to the accompanying drawings.

An apparatus for transmitting/receiving multiple codewords in an SC-FDMAsystem according to an exemplary embodiment of the present inventionincludes a transmitter that generates multiple codewords for user dataand transmits the generated multiple codewords in the SC-FDMA system anda receiver that receives the multiple codewords from the transmittingapparatus and sequentially performs decoding and a successiveinterference cancellation (SIC) process on the received multiplecodewords. FIG. 1 shows the overall structure of the transmitter, andFIG. 7 shows the overall structure of the receiver.

The transmitter divides user data into M sub-streams, independentlyperforms channel coding and QAM (quadrature amplitude modulation)mapping on each of the sub-streams, maps each signal that has beensubjected to QAM mapping to a DFT index, performs DFT on each signalthat is mapped to the DFT index, and transmits multiple codewords.

The receiver receives the multiple codewords from the transmitter,sequentially performs detecting and decoding on each of the sub-streams,generates a reception signal for the decoded sub-stream, and subtractsthe reception signal for the decoded sub-stream from the receivedmultiple codewords.

In the apparatus for transmitting and receiving the multiple codewordsin the SC-FDMA system according to the exemplary embodiment of thepresent invention, the transmitter divides one user signal into severalsub-streams and simultaneously transmits several sub-streams, and thereceiver performs the successive interference cancellation process usingthe decoded sub-streams. As a result, it is possible to effectivelyremove interference between symbols, as compared to a method oftransmitting one sub-stream. That is, the apparatus fortransmitting/receiving the multiple codewords in the SC-FDMA systemaccording to the exemplary embodiment of the invention can considerablyreduce interference between symbols in a frequency selective fadingenvironment, while minimizing a PAPR.

FIG. 1 is a block diagram illustrating the structure of a transmitter towhich multiple codewords is applied in a communication system usingSC-FDMA according to the exemplary embodiment of the present invention.Particularly, FIG. 1 shows the structure of a transmitter fortransmitting multiple codewords for data input from an arbitrary user kin the SC-FDMA system.

As shown in FIG. 1, a transmitter of a communication system usingSC-FDMA (hereinafter referred to as an “SC-FDMA transmitter”) accordingto an exemplary embodiment of the present invention includes ademultiplexer 110, a plurality of encoders 130-1 to 130-M, a pluralityof QAM mappers 140-1 to 140-M, a DFT mapper 150, an L-point DFT 160, asubcarrier mapper 170, and an N-point IFFT 180. Alternatively, theSC-FDMA transmitter according to an exemplary embodiment of the presentinvention may further include a rate controller 120.

The demultiplexer 110 demultiplexes data input from the user k into Msub-streams, and inputs the sub-streams to the corresponding encoders130-1 to 130-M.

The rate controller 120 determines a data transfer rate of eachsub-stream using information (i.e., channel state information CQI₁ toCQI_(M)) that is fed back from a receiver of the communication systemusing SC-FDMA according to the exemplary embodiment of the presentinvention (hereinafter referred to as an “SC-FDMA receiver”). The ratecontroller 120 determines a code rate and a modulation method for eachsub-stream using the channel state information items CQI₁ to CQI_(M) forthe M sub-streams that are fed back from the SC-FDMA receiver. Then, therate controller 120 inputs the determined code rates to thecorresponding encoders 130-1 to 130-M, and inputs the determinedmodulation methods to the corresponding QAM mappers 140-1 to 140-M.Alternatively, the rate controller 120 calculates an SINR (signal tointerference and noise ratio) for each sub-stream using the informationthat is fed back from the SC-FDMA receiver, and determines the datatransfer rate on the basis of the calculated SINR for each sub-stream.Alternatively, the rate controller 120 controls the encoders 130-1 to130-M and the QAM mappers 140-1 to 140-M to perform channel coding andQAM symbol mapping on the sub-streams on the basis of the data transferrate (or SINR) that is fed back from the SC-FDMA receiver, anddetermines transmission power for each sub-stream on the basis of thetransmission power P₁ of the first sub-stream that is fed back from theSC-FDMA receiver and a transmission power offset Ω.

The encoders 130-1 to 130-M independently perform channel coding on thesub-streams input from the demultiplexer 110. Alternatively, theencoders 130-1 to 130-M receive the code rates from the rate controller120, perform channel coding on the corresponding sub-streams input fromthe demultiplexer 110 on the basis of the received code rates, and inputthe sub-streams of which channels are coded to the QAM mappers 140-1 to140-M, respectively.

The QAM mappers 140-1 to 140-M independently perform QAM mapping on thesub-streams input from the encoders 130-1 to 130-M. Alternatively, theQAM mappers 140-1 to 140-M receive the modulation methods determined bythe rate controller 120, uses the received modulation methods to performQAM mapping on the sub-streams input from the encoders 130-1 to 130-M,and input sub-stream Nos. 1 to M that are subjected to QAM mapping tothe DFT mapper 150, respectively.

The DFT mapper 150 receives the M sub-streams (1 to M) that have beensubjected to QAM mapping from the QAM mappers 140-1 and 140-M, maps thereceived M sub-streams (1 to M) to DFT indexes, and inputs the mappedDFT indexes to the L-point DFT 160. At that time, the DFT mapper 150allocates fixed DFT indexes to the M sub-streams (1 to M) input from theQAM mappers 140-1 to 140-M. Alternatively, the DFT mapper 150 changesthe DFT indexes allocated to the M sub-streams (1 to M) that arerespectively input from the QAM mappers 140-1 to 140-M with time toallocate all the DFT indexes at the same (or similar) ratio.

The L-point DFT 160 receives the DFT indexes mapped by the DFT mapper150, performs L-point DFT on the received DFT indexes, and inputs thesignals obtained by the L-point DFT to the subcarrier mapper 170.

The subcarrier mapper 170 receives the signals obtained by the L-pointDFT from the L-point DFT 160, maps the received signals to subcarrierscorresponding to the user k, and inputs the mapped subcarriers to theN-point IFFT 180.

The N-point IFFT 180 receives the mapped subcarriers from the subcarriermapper 170, performs N-point IFFT on the received subcarriers, andtransmits the signals obtained by the N-point IFFT through atransmitting antenna.

Next, a transmitting method using multiple codewords in a communicationsystem using SC-FDMA according to an exemplary embodiment of the presentinvention will be described in detail with reference to FIGS. 2 to 6.FIG. 2 is a flowchart illustrating the transmitting method usingmultiple codewords in a communication system using SC-FDMA according tothe exemplary embodiment of the present invention. FIG. 3 is a diagramillustrating an example of a mapping method when M=L in the DFT mapperaccording to an exemplary embodiment of the present invention. FIG. 4 isa diagram illustrating another example of the mapping method when M=L inthe DFT mapper according to the exemplary embodiment of the presentinvention. FIG. 5 is a diagram illustrating an example of a mappingmethod when M=L/2 in the DFT mapper according to the exemplaryembodiment of the present invention. FIG. 6 is a diagram illustratinganother example of the mapping method when M=L/2 in the DFT mapperaccording to the exemplary embodiment of the present invention.

In the SC-FDMA transmitter of the communication system, thedemultiplexer 110 demultiplexes a data stream for the user k into Msub-streams (step S210). In this case, the demultiplexer 110demultiplexes data for the user k into M sub-streams, and inputs thesub-streams to the corresponding encoders 130-1 to 130-M.

The M encoders 130-1 to 130-M and the M QAM mappers 140-1 to 140-Mindependently perform channel coding and QAM mapping on the Msub-streams, respectively (step S220). In this case, the code rates andthe modulation methods for the M sub-streams are determined on the basisof the channel state information items CQI₁ to CQI_(M) for the Msub-streams that are fed back from the SC-FDMA receiver.

That is, the rate controller 120 determines the code rates and themodulation methods for the M sub-streams on the basis of the channelstate information items CQI₁ to CQI_(M) for the M sub-streams that arefed back from the SC-FDMA receiver. Then, the rate controller 120 inputsthe determined code rates to the encoders 130-1 to 130-M, and inputs thedetermined modulation methods to the QAM mappers 140-1 to 140-M.

Then, the encoders 130-1 to 130-M perform channel coding on thesub-streams input from the demultiplexer 110 according to the code ratesinput from the rate controller 120, and inputs the sub-streams of whichchannels are coded to the QAM mappers 140-1 to 140-M. The QAM mappers140-1 to 140-M use the modulation methods input from the rate controller120 to perform QAM mapping on the sub-streams input from the encoders130-1 to 130-M, respectively, and input the M sub-streams (1 to M) thathave been subjected to QAM mapping to the DFT mapper 150.

Then, the DFT mapper 150 maps the M sub-streams (1 to M) that have beensubjected to QAM mapping in the QAM mappers 140-1 to 140-M to DFTindexes (step S230). That is, the DFT mapper 150 receives the Msub-streams (1 to M) that have been subjected to QAM mapping from theQAM mappers 140-1 to 140-M, maps the received M sub-streams (1 to M) tothe DFT indexes, and inputs the mapped DFT indexes to the L-point DFT160. In step S230 in which the signals that have been subjected to QAMmapping are mapped to the DFT indexes, fixed DFT indexes are allocatedto the M sub-streams (1 to M) at all times. Alternatively, in step S230in which the signals that have been subjected to QAM mapping are mappedto the DFT indexes, the DFT indexes allocated to the M sub-streams (1 toM) are changed with time such that all of the DFT indexes are allocatedat the same (or similar) ratio.

Then, the DFT mapper 150 performs DFT on the signals mapped to the DFTindexes (step S240). The L-point DFT 160 receives the mapped DFT indexesfrom the DFT mapper 150, performs L-point DFT on the received DFTindexes, and inputs the signals obtained by the L-point DFT to thesubcarrier mapper 170.

The subcarrier mapper 170 maps the signals subjected to DFT to thesubcarriers corresponding to the user k (step S250). In this case, thesubcarrier mapper 170 receives the signals obtained by the L-point DFT,maps the received signals to the subcarriers corresponding to the userk, and inputs the mapped subcarriers to the N-point IFFT 180.

The subcarrier mapper 170 performs IFFT on the mapped subcarriers, andtransmits the transformed signals through the transmitting antenna (stepS260). The N-point IFFT 180 receives the subcarriers mapped by thesubcarrier mapper 170, performs N-point IFFT on the receivedsubcarriers, and transmits the signals obtained by the N-point IFFTthrough the transmitting antenna.

Next, the mapping method in the DFT mapper 150 in step S230 will bedescribed.

First, the QAM mapper 140-M performs QAM mapping on an m-th sub-streamof data for the user k to obtain a signal d_(k) ^((m)) that is definedby Equation 1 given below. In this case, it is assumed that onesub-stream is composed of Q QAM signals.d _(k) ^((m)) =[d _(k) ^((m))(0),d _(k) ^((m))(1), . . . ,d _(k)^((m))(Q−1)],m=1,2, . . . ,M  (Equation 1)

Therefore, an output signal s_(k)(n) of the DFT mapper 150 at an L-pointfor an n-th transmission time is defined by Equation 2 given below:s _(k)(n)=[s _(k,0)(n),s _(k,1)(n), . . . ,s _(k,L-1)(n)].  (Equation 2)

Various methods can be used to map d_(k) ^((m)), m=1, 2, . . . M tos_(k)(n). For example, the following mapping method can be used. WhenM=L, d_(k) ^((l))(n) is mapped to one of the L elements formings_(k)(n). When M=L/P (where P is a positive integer), P elements among Qelements forming d_(k) ^((m)) are mapped to P elements among L elementsforming s_(k)(n).

FIGS. 3 and 4 are diagrams illustrating examples of the mapping methodperformed by the DFT mapper 150 when M=L, L=4, and Q=6. FIG. 3 shows anexample of the mapping of one sub-stream to one DFT index, and FIG. 4shows an example of the mapping of one sub-stream to another DFT indexwith time. FIGS. 5 and 6 are diagrams illustrating examples of themapping method performed by the DFT mapper 150 when M=L/2, L=4, and Q=6.In this exemplary embodiment of the present invention, the mappingmethods shown in FIGS. 3, 4, 5, and 6 are described, but the presentinvention is not limited thereto. Various mapping methods other than themapping methods shown in FIGS. 3, 4, 5, and 6 may be used.

The L-point DFT 160 performs L-point DFT on the output signal s_(k)(n)of the DFT mapper 150, and outputs a signal x_(k)(n) that is defined byEquation 3 given below:

$\begin{matrix}{{{x_{k}(n)} = {{Vs}_{k}(n)}},{n = 0},1,\ldots\mspace{14mu},{\frac{QM}{L} - 1.}} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$

In Equation 3, “V=[v₁ v₂ . . . v_(L)]” is a DFT matrix, and an element[V]_(l,m), which is in an l-th row and an m-th column, is defined byEquation 4 given below:

$\begin{matrix}{\lbrack V\rbrack_{l,m}\frac{1}{\sqrt{2\;\pi\; L}}{{\mathbb{e}}^{{- {{j2\pi}{lm}}}/L}.}} & \left( {{Equation}\mspace{14mu} 4} \right)\end{matrix}$

Next, the SC-FDMA receiver for a communication system using SC-FDMAaccording to an exemplary embodiment of the present invention will bedescribed in detail with reference to FIGS. 7 and 8.

FIG. 7 is a diagram illustrating the structure of the SC-FDMA receiveraccording to the exemplary embodiment of the present invention. TheSC-FDMA receiver according to the exemplary embodiment of the presentinvention performs N-point FFT on a received signal, performs thedemapping of a subcarrier to separate user signals, and detects anddecodes the separated user signals to estimate each of the user signals.

As shown in FIG. 7, the SC-FDMA receiver according to the exemplaryembodiment of the present invention includes an N-point FFT 710, asubcarrier demapper 720, and a plurality of detectors and decoders 730-1to 730-J. The N-point FFT 710 receives signals through a receivingantenna, performs N-point FFT on the received signals, and inputs thesignals subjected to the N-point FFT to the subcarrier demapper 720. Thesubcarrier demapper 720 performs a subcarrier demapping process on thesignals input from the N-point FFT 710 to separate user signals, andinputs the separated user signals to the corresponding detectors anddecoders 730-1 to 730-J. The detectors and decoders 730-1 to 730-Jdetect and decode the user signals input from the subcarrier demapper720 to estimate the user signals.

FIG. 8 is a diagram illustrating the structure of an SC-FDMA receiverfor a user k according to another exemplary embodiment of the presentinvention. Particularly, FIG. 8 is a detailed diagram illustrating astructure for detecting and decoding signals for an arbitrary user k.

As shown in FIG. 8, the SC-FDMA receiver for the user k according toanother exemplary embodiment of the present invention includes anN-point FFT 810, a subcarrier demapper 820, a detector 830, a decoder840, a CRC (cyclic redundancy checker) 850, a signal regenerator 860,and an SIC 870. Alternatively, the SC-FDMA receiver for the user kaccording to another exemplary embodiment of the present invention mayfurther include a channel state information generator 880. The N-pointFFT 810 and the subcarrier demapper 820 have the same structures as theN-point FFT 710 and the subcarrier demapper 720 shown in FIG. 7, andthus a detailed description thereof will be omitted.

The detector 830 performs a channel estimating process on the usersignals input from the subcarrier demapper 820, detects the usersignals, and inputs Q detected output signals forming one sub-stream tothe decoder 840.

The decoder 840 decodes the Q detected output signals input from thedetector 830, and inputs the decoded bits to the CRC 850.

The CRC 850 receives the decoded bits from the decoder 840, and checksthe CR (cyclic redundancy) values thereof. When the checked CR value iscorrect, the CRC 850 considers that signals for one sub-stream arecorrectly decoded, and notifies the signal regenerator 860 of the fact.On the other hand, when the checked CR value is incorrect, the CRC 850determines that the signals for one sub-stream are incorrectly decoded,and transmits a retransmission request signal to the SC-FDMAtransmitter.

The signal regenerator 860 receives the notification from the CRC 850,regenerates the original transmission signal for one sub-stream, andinputs the regenerated signal to the SIC 870.

The SIC 870 multiplies the transmission signal input from the signalregenerator 860 by a channel value to generate a reception signal forone sub-stream, subtracts a received sub-stream signal from a userreception signal to obtain a modulated user reception signal, and inputsthe modulated user reception signal to the detector 830 again to detectother sub-stream signals.

The channel state information generator 880 calculates channel stateinformation values for M sub-streams and feeds back the channel stateinformation values to the SC-FDMA transmitter. In this case, the channelstate information values for the M sub-streams of the user k areobtained by calculating the SINR values of the signals detected by thedetector 830, considering sequential interference cancellation.Alternatively, the channel state information generator 880 calculatesthe transmission power P₁, transmission power offset Ω, and datatransfer rate R of a first sub-stream, and feeds back the calculatedtransmission power P₁, transmission power offset Ω, and data transferrate R of the first sub-stream to the SC-FDMA transmitter.

Next, a receiving method using multiple codewords in a communicationsystem using SC-FDMA according to an exemplary embodiment of the presentinvention will be described in detail with reference to a flowchartshown in FIG. 9.

The SC-FDMA receiver in the communication system using SC-FDMA performsFFT on a received signal (step S910). In this case, the N-point FFT 810performs N-point FFT on the signal received through the receivingantenna, and inputs the signals subjected to the N-point FFT to thesubcarrier demapper 720.

Then, the subcarrier demapper 820 performs a subcarrier demappingprocess on the signals output from the N-point FFT 810 to extract usersignals (step S920). Specifically, the subcarrier demapper 820 performsthe subcarrier demapping process on the signals input from the N-pointFFT 810 to separate the user signals, and inputs the separated usersignals to the detector 830.

Then, the detector 830 detects the user signals for one sub-streamseparated by the subcarrier demapper 820 (step S930). Specifically, thedetector 830 detects the user signals input from the subcarrier demapper820, and inputs Q detected output signals forming one sub-stream to thedecoder 840.

Subsequently, decoding is performed using the Q detected output signalsforming one sub-stream (step S940). Specifically, the decoder 840decodes the Q detected output signals input from the detector 830, andinputs the decoded signals to the CRC 850.

Steps S930 and S940 in which signals for an arbitrary user k aredetected and decoded will be described in more detail below.

In order to estimate signals for the user k, the detector 830 extractsonly the signals for the user k that are input from the subcarrierdemapper 820. When the extracted signal for the user k is “r_(k)(n)”,the extracted signal r_(k)(n) can be represented by Equation 5 givenbelow:r _(k)(n)=H _(k) Vs _(k)(n)+w _(k)(n)  (Equation 5)

where “H_(k)=diag(H_(k,0),H_(k,1), . . . , H_(k,L-1))” indicates afrequency domain channel characteristic for the user k, and “w_(k)(n)”indicates an AWGN (additive white Gaussian noise) signal having anaverage of “0” and a variance of “σ_(w) ²”.

The detector 830, the decoder 840, and the SIC 850 are used to separateM sub-streams from the extracted signal r_(k)(n). It is assumed thatdecoding is sequentially performed on the M sub-streams in the order ofsub-streams 1, 2, . . . , M, for convenience of explanation. However,the present invention is not limited thereto.

First, the detector 830 is used to decode the first sub-stream separatedfrom the extract signal r_(k)(n). In this exemplary embodiment of thepresent invention, any type of detector may be used, but an MMSE(minimum mean square error) detector is used in this exemplaryembodiment of the present invention for convenience of explanation.

The MMSE detector for finding out an element s_(k,l)(n), which is anl-th element of s_(k)(n), from the extracted signal r_(k)(n) can berepresented by Equation 6 given below:

where “v_(l)” indicates an l-th column vector of a DFT matrix (v),f _(k) ⁽¹⁾=(H _(k) H _(k) ^(H)+σ_(w) ² I _(L))⁻¹ H _(k) v_(l).  (Equation 6)

When M=L and DFT mapping is performed as shown in FIG. 3, f_(k) ⁽¹⁾ isused to detect the first sub-stream. When DFT mapping is performed asshown in FIG. 5A, f_(k) ⁽¹⁾ and f_(k) ⁽²⁾ are used to detect the firstsub-stream. When DFT mapping is performed as shown in FIG. 5B, f_(k) ⁽¹⁾and f_(k) ⁽³⁾ are used to detect the first sub-stream. In the SC-FDMAtransmitter, the MMSE detector corresponding to the first sub-stream isused to perform DFT mapping on the first sub-stream, and the Q detectedoutput signals forming the first sub-stream are decoded.

The decoder 840 determines a transmission signal for the sub-streamusing the decoded signal (step S950). The CRC 850 checks the CR value ofthe signal decoded by the decoder 840. When the CR value is correct, theCRC 850 considers that the signal for the first sub-stream is correctlydecoded, and notifies the signal regenerator 860 of the fact. On theother hand, when the CR value is incorrect as the check result, the CRC850 considers that the signal for the first sub-stream is incorrectlydecoded, and transmits a retransmission request signal to the SC-FDMAtransmitter.

When the signal for the first sub-stream is correctly decoded, thesignal regenerator 860 regenerates the original transmission signal forthe first sub-stream and inputs the regenerated signal to the SIC 870.

The SIC 870 multiplies the transmission signal generated by the signalregenerator 860 by a channel value to obtain a reception signal for thesub-stream (step S960). Specifically, the SIC 870 multiplies thetransmission signal input from the signal regenerator 860 by a channelvalue to generate a reception signal for the first sub-stream, andsubtracts the reception signal generated by the signal regenerator 860from a detected input signal r_(k)(n) to generate a modulated detectioninput signal r_(k) ⁽¹⁾(n) (step S970). That is, the SIC 870 subtractsthe reception signal input from the signal regenerator 860 from theextracted signal r_(k)(n) to generate a modulated signal r_(k) ⁽¹⁾(n),and inputs the modulated signal r_(k) ⁽¹⁾(n) to the detector 830 again.

Then, the same method as that used for the first sub-stream (i.e., theprocesses from step S930 to step S970) is performed on r_(k) ⁽¹⁾(n) todecode the second sub-stream. This method (i.e., processes from stepS930 to step S970) is similarly performed on the sub-streams (3, 4, . .. , M) to decode the sub-streams, thereby estimating the sub-stream (2,3, . . . , M).

That is, it is checked that the processes from step S930 to step S970are repeated up to the M-th sub-stream (step S980). As the check result,when the processes are not performed up to the M-th sub-stream, theprocesses are repeated from step S930. The processes from step S930 tostep S970 are repeatedly performed on the sub-streams (2, 3, . . . , M)to estimate the sub-streams (2, 3, . . . , M).

Another embodiment of the present invention provides a method ofcontrolling the transfer rate and the transmission power of eachsub-stream using feedback information output from the SC-FDMA receiverin the SC-FDMA transmitter, which makes it possible to improve a datatransfer rate. In addition, according to this embodiment of the presentinvention, the SC-FDMA receiver feeds back a post-detection SINR foreach sub-stream to the SC-FDMA transmitter, and the SC-FDMA transmitterindependently performs channel coding and QAM mapping on eachsub-stream, which makes it possible to improve an SIC gain and a datatransfer rate by the channel state adaptive transmission.

Next, a method of feeding back channel state information items CQI₁ toCQI_(M) for M sub-streams to the SC-FDMA transmitter will be described.

As described above, the SC-FDMA receiver estimates channels for Msub-streams, calculates a post-detection SINR for each sub-stream inconsideration of SIC, and feeds back the calculated SINR for eachsub-stream to the SC-FDMA transmitter.

That is, when the signal r_(k)(n) for the user k is extracted from thesubcarrier demapper 820 and the sub-streams (1, 2, . . . , M) areremoved from the extracted signal to obtain a modulated signal, themodulated signal is defined as “r_(k) ^((m))(n)” and it is defined that“r_(k)(n)=r_(k) ⁽⁰⁾(n)”. The SINR value of the signal obtained by thedetector 830 may be used as the channel state information value. Whenthe detector 830 detects a modified signal r_(k) ^((m-1))(n) for theM-th sub-stream to obtain a signal, a channel state information valuefor the M-th sub-stream of the user k can be obtained by calculating theSINR value of the obtained signal. The SINR is a post-detection SINR inconsideration of SIC. In this way, the channel state information valuesfor M sub-streams are calculated, and the calculated values are fed backto the SC-FDMA transmitter.

Then, the SC-FDMA transmitter determines a data transfer rate for eachsub-stream using the feedback information (i.e., channel stateinformation) from the SC-FDMA receiver.

In the DFT mapping method, different DFT indexes are mapped to onesub-stream.

However, when DFT indexes are mapped to the corresponding sub-streams,the difference between the post-detection SINRs of adjacent sub-streamsis substantially constant. That is, when the post-detection SINR of aj-th sub-stream is referred to as “SINR_(j)”, the sub-streams (1, 2, . .. , M−1) satisfy Equation 7 given below:SINR_(j+1)≈SINR_(j)+Δ.  (Equation 7)

Therefore, when the DFT indexes are mapped to the correspondingsub-streams, the SC-FDMA receiver may feed back an SINR offset Δ betweenthe post-detection SINR of the j-th sub-stream (SINR) and thepost-detection SINR of the first sub-stream.

That is, as described above, the SC-FDMA receiver estimates channels forM sub-streams, calculates the post-detection SINR of the firstsub-stream (SINR₁), calculates the SINR offset Δ between thesub-streams, and feeds back the calculated post-detection SINR of thefirst sub-stream (SINR₁) and the calculated SINR offset Δ to the SC-FDMAtransmitter.

In this case, the SC-FDMA transmitter calculates an SINR for eachsub-stream using the information fed back from the SC-FDMA receiver, andthe SINR of the j-th sub-stream is estimated by Equation 8 given below.SINR_(j)≈SINR₁+(j−1)Δ.  (Equation 8)

In Equation 8, “SINR₁” indicates the post-detection SINR of the j-thsub-stream in consideration of sequential interference cancellation, andthe SC-FDMA transmitter determines a data transfer rate according to thecalculated SINR for each sub-stream and transmits data at the determineddata transfer rate.

In this embodiment, a channel feedback method when all of thesub-streams are transmitted with the same transmission power has beendescribed. Next, the structure in which SC-FDMA transmitter transmitsthe sub-streams at the same data transfer rate but with differenttransmission powers will be described.

The SC-FDMA receiver feeds back two values to the SC-FDMA transmitter.One of the two values is a data transfer rate (or SINR) applied to allof the sub-streams, and the other value is the transmission power offsetΩ.

That is, as described above, the SC-FDMA receiver estimates channels forM sub-streams, calculates the transmission power P₁, transmission poweroffset Ω, and data transfer rate R of the first sub-stream, and feedsback the calculated transmission power P₁, transmission power offset Ω,and data transfer rate R of the first sub-stream to the SC-FDMAtransmitter.

The SC-FDMA transmitter performs channel coding and QAM symbol mappingon each sub-stream using the data transfer rate (or SINR) from theSC-FDMA receiver, and calculates power required to transmit eachsub-stream using the transmission power offset Ω. The SC-FDMAtransmitter performs channel coding and QAM mapping using the datatransfer rate R that is fed back from the SC-FDMA receiver, anddetermines power required to transmit each sub-stream, on the basis ofthe transmission power P₁ and transmission power offset Ω of the firstsub-stream that are fed back from the SC-FDMA receiver, by usingEquation 9 given below:P _(j) =P ₁−(j−1)Ω,  (Equation 9)

where “P_(j)” indicates power required to transmit the j-th sub-stream.

In this way, when the sub-streams are transmitted by different powers,it is possible to considerably reduce the reception error rate.

The components described in the exemplary embodiments of the presentinvention may be achieved by hardware components including at least oneDSP (digital signal processor), a processor, a controller, an ASIC(application specific integrated circuit), a programmable logic elementsuch as an FPGA (field programmable gate array), other electronicdevices, and combinations thereof. At least some of the functions or theprocesses described in the exemplary embodiments of the presentinvention may be achieved by software, and the software may be recordedon a recording medium. The components, the functions, and the processesdescribed in the exemplary embodiments of the present invention may beachieved by a combination of hardware and software.

While this invention has been described in connection with what ispresently considered to be practical exemplary embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

According to the above-described exemplary embodiments of the presentinvention, it is possible to minimize PAPR in a communication systemusing SC-FDMA, and considerably reduce interference between symbols in afrequency selective fading environment.

What is claimed is:
 1. A method of transmitting symbols in atransmitter, the method comprising: preparing a plurality of sub-streamsincluding a first sub-stream and a second sub-stream; independentlyperforming channel coding on each of the first and second sub-streams;mapping the channel coded sub-streams to modulation symbols, whereindifferent channel coded sub-streams are mapped to different modulationsymbols; generating single carrier frequency division multiple access(SC-FDMA) symbols from the modulation symbols, one of the SC-FDMAsymbols being generated by using at least part of the modulation symbolsfrom the first sub-stream and part of the modulation symbols from thesecond sub-stream; and transmitting the SC-FDMA symbols to a receiver.2. The method of claim 1, wherein generating the SC-FDMA symbolscomprises: transforming the modulation symbols; mapping the transformedsymbols to subcarriers; and generating the SC-FDMA symbols from thesymbols mapped to the subcarriers.
 3. The method of claim 1, wherein atransfer rate of the first sub-stream and a transfer rate of the secondsub-stream are differently determined.
 4. The method of claim 3, furthercomprising receiving an offset for determining the transfer rate of thefirst sub-stream and the transfer rate of the second sub-stream from thereceiver.
 5. The method of claim 1, further comprising receivingfeedback information from the receiver; and determining transfer ratesof the sub-streams based on the feedback information.
 6. A method ofreceiving a signal in a receiver, the method comprising: receivingsingle carrier frequency division multiple access (SC-FDMA) symbols froma transmitter; and extracting a plurality of sub-streams including afirst sub-stream and a second sub-stream from the SC-FDMA symbols,wherein the SC-FDMA symbols are generated, by the transmitter, byindependently performing channel coding on each of the first and secondsub-streams, mapping the channel coded sub-streams to modulationsymbols, generating the SC-FDMA symbols from the modulation symbols,wherein different channel coded sub-streams are mapped to differentmodulation symbols, and wherein one of the SC-FDMA symbols are generatedby using at least part of the modulation symbols from the firstsub-stream and part of the modulation symbols from the secondsub-stream.
 7. The method of claim 6, wherein the modulation symbols aretransformed, the transformed symbols are mapped to subcarriers, and theSC-FDMA symbols are generated from the symbols mapped to thesubcarriers, by the transmitter.
 8. The method of claim 6, wherein atransfer rate of the first sub-stream and a transfer rate of the secondsub-stream are differently determined.
 9. The method of claim 8, furthercomprising transmitting an offset for determining the transfer rate ofthe first sub-stream and the transfer rate of the second sub-stream. 10.The method of claim 6, further comprising transmitting feedbackinformation for determining transfer rates of the sub-streams to thetransmitter.
 11. A transmitter comprising: a demultiplexer configured toprepare a plurality of sub-streams including a first sub-stream and asecond sub-stream; a plurality of encoders configured to independentlyperform channel coding on each of the first and second sub-streams; amapper configured to map the channel coded sub-streams to modulationsymbols, wherein different channel coded sub-streams are mapped todifferent modulation symbols; and a generator configured to generatesingle carrier frequency division multiple access (SC-FDMA) symbols, tobe transmitted to a receiver, from the modulation symbols, one of theSC-FDMA symbols being generated by using at least part of the modulationsymbols from the first sub-stream and part of the modulation symbolsfrom the second sub-stream.
 12. The transmitter of claim 11, wherein thegenerator comprises: a first transformer configured to transform themodulation symbols; a subcarrier mapper configured to map thetransformed symbols to subcarriers; and a second transformer configuredto generate the SC-FDMA symbols from the symbols mapped to thesubcarriers.
 13. The transmitter of claim 11, wherein a transfer rate ofthe first sub-stream and a transfer rate of the second sub-stream aredifferently determined.
 14. The transmitter of claim 13, furthercomprising a rate controller configured to receive an offset fordetermining the transfer rate of the first sub-stream and the transferrate of the second sub-stream from the receiver.
 15. The transmitter ofclaim 11, further comprising a rate controller configured to receivefeedback information from the receiver, and determine transfer rates ofthe sub-streams based on the feedback information.
 16. A receivercomprising: a receiver configured to receive single carrier frequencydivision multiple access (SC-FDMA) symbols from a transmitter; and adecoder configured to extract a plurality of sub-streams including afirst sub-stream and a second sub-stream from the SC-FDMA symbols,wherein the SC-FDMA symbols are generated, by the transmitter, byindependently performing channel coding on each of the first and secondsub-streams, mapping the channel coded sub-streams to modulationsymbols, generating the SC-FDMA symbols from the modulation symbols,wherein different channel coded sub-streams are mapped to differentmodulation symbols, and wherein one of the SC-FDMA symbols are generatedby using at least part of the modulation symbols from the firstsub-stream and part of the modulation symbols from the secondsub-stream.
 17. The receiver of claim 16, wherein the modulation symbolsare transformed, the transformed symbols are mapped to subcarriers, andthe SC-FDMA symbols are generated from the symbols mapped to thesubcarriers, by the transmitter.
 18. The receiver of claim 16, wherein atransfer rate of the first sub-stream and a transfer rate of the secondsub-stream are differently determined.
 19. The receiver of claim 18,further comprising an information generator configured to transmit anoffset for determining the transfer rate of the first sub-stream and thetransfer rate of the second sub-stream.
 20. The receiver of claim 16,further comprising an information generator configured to transmitfeedback information for determining transfer rates of the sub-streamsto the transmitter.